A plate-shaped gas sensor element including a plurality of ceramic layers layered together has been known as a gas sensor element used for a gas sensor (see, for example, Japanese Patent Applications Laid-Open (kokai) No. 2010-261727, No. 2003-185622 and No. 2002-340845). Such a gas sensor element includes a solid electrolyte ceramic layer as one of the ceramic layers. Electrodes for detecting a specific gas component contained in an object gas (gas to be to be measured) are formed on the surfaces of the solid electrolyte ceramic layer. A gap for leading a reference gas or the object gas to the corresponding electrode is formed in the gas sensor element. In general, a gas sensor has a heater, and the electric power necessary for heating by the heater can be decreased by reducing the size of the gas sensor.